The field of the invention is magnetic resonance imaging (MRI) and in particular a method for reducing image artifacts using an RF preparatory pulse which is shifted in frequency from a nominal Larmor frequency corresponding to the spin species being imaged.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of a magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic frequency that is termed the Larmor frequency, f0, and which is dependent on the strength of the magnetic field and on the gyromagnetic constant γ of the nucleus: i.e., f0=γB, where γ=42.56 MHz/T for hydrogen nuclei, and B is the strength of the magnetic field. Hydrogen (H1) is the spin species of choice for most MRI applications and for example, the Larmor frequency f0 for hydrogen nuclei in a 1.5 T magnetic field is 63.8 MHz.
MRI takes advantage of this phenomenon by subjecting an object to be imaged (such as human tissue) to a uniform magnetic field (polarizing field B0) along a z direction, and then subjecting the object to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency such that the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. After the excitation signal B1 (RF excitation pulse) is terminated, a nuclear magnetic resonance (NMR) signal is emitted by the excited spins and this signal is detected.
In MR systems, the excited spins (typically hydrogen) induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the transverse magnetic moment Mt. The amplitude, A, of the emitted NMR signal decays in an exponential fashion with time, t. The decay constant 1/T*2 depends on the homogeneity of the magnetic field and on T2, which is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B1 in a perfectly homogeneous field. The practical value of the T2 constant is that tissues have different T2 values and this can be exploited as a means of enhancing the contrast between such tissues.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process that is characterized by the time constant T1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T1 time constant is longer than T2, much longer in most substances of medical interest. As with the T2 constant, the difference in T1 between tissues can be exploited to provide image contrast.
When utilizing the received NMR signals to produce images, it is necessary to elicit NMR signals from specific locations in the subject, which is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The resulting set of received NMR signals can be digitized and processed to reconstruct an image of the object using one of many well known reconstruction techniques.
High field MRI, with B0 field strengths of 3 T or higher, is rapidly winning acceptance in both clinical and research programs. High field MRI offers many benefits, while simultaneously presenting many research and design problems. The main benefit of high field MRI is increased signal to noise ratio (SNR), which increases linearly with static field strength. This increase provides significant advantages in terms of spatial, temporal, and spectral resolution. However, specific absorption rates (SAR) limits are imposed which restrict the amount of RF applied to a subject, and these SAR limits are more easily exceeded with high magnetic field strengths.
The time required to acquire sufficient NMR signals to reconstruct an image is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences that have a very short repetition time (TR) and result in complete scans that can be conducted in seconds rather than minutes.
The concept of acquiring NMR imaging data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a series of gradient-recalled NMR echo signals for each RF excitation pulse. These NMR signals are separately phase encoded so that a set of views sufficient to reconstruct an image can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging (“EPI”) are well known.
A variant of the echo-planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al. in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled “RARE Imaging: A Fast Imaging Method for Clinical MR.” The primary difference between the RARE sequence and the EPI sequence lies in the manner in which NMR echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes.
Both of these “fast spin echo” imaging methods involve the acquisition of multiple echo signals from a single RF excitation pulse in which each acquired echo signal is separately phase encoded. Each pulse sequence, or “shot”, therefore results in the acquisition of a plurality of views and single shot scans are commonly employed with the EPI method. However, a plurality of shots is typically employed to acquire a complete set of image data when the RARE fast spin echo sequence is employed. For example, a RARE pulse sequence might acquire 8 or 16 separate echo signals per shot, and an image requiring 256 views would, therefore, require 32 or 16 shots, respectively.
Pulse sequences based on spin echo, RARE, and EPI often employ a two stage pulse sequence with a preparatory stage followed by a time delay prior to an imaging stage. One such pulse sequence is referred to as an inversion recovery (IR) pulse sequence. Conceptually, the first or preparatory stage, referred to as the “IR module,” includes an RF preparatory or inversion pulse, an optional spoiler gradient, and any slice-selection gradient (should the RF preparatory pulse be non-selective). The second or imaging stage of the IR pulse sequence, referred to as the “host sequence,” begins with an RF excitation pulse after a time delay referred to as an inversion time (TI) from the RF preparatory pulse, and typically includes a self-contained pulse sequence, such as a spin-echo sequence, gradient echo sequence, RARE sequence, EPI sequence, or the like.
Spin echo, RARE, and EPI pulse sequences often include an IR module for each host sequence. However, when fast gradient echo sequences are employed, the short TR does not allow time for a full IR module to be included before every host sequence. As described by J. P. Mugler et al in “Three-Dimensional Magnetization-Prepared Rapid Gradient-Echo Imaging (3D MP RAGE),” Magnetic Resonance In Medicine 15, 152-157 (1990); by M. Brant-Zawadzki in “MP RAGE: A Three-Dimensional, T1-Weighted, Gradient-Echo Sequence—Initial Experience in the Brain,” Radiology 1992; 182: 769-775; and by J. P. Mugler et al. in “T2-Weighted Three-Dimensional MP-RAGE MR Imaging,” JMRI 1991:1:731-737; a plurality of gradient-echo pulse sequences can be performed after each IR module. In particular, for T1-weighted imaging, a non-selective RF preparatory pulse (having an angle selected from 0 to 180 degrees) is applied and followed by a TI interval. After the TI interval, a series of fast gradient-recalled echo sequences are performed to acquire a corresponding series of phase-encoded lines in k-space. Following a recovery period, the process is repeated as necessary to fully sample k-space.